Passive optical network

ABSTRACT

A passive optical network includes a head-end station and a plurality of terminations. The terminations are responsive to control signals from the head-end station. The head-end station is arranged to receive frames having a predetermined bit rate and comprising signals transmitted by the terminations. Ranging pulses are transmitted from each of the terminations to the head-end station where the received ranging pulses are sampled at a high sampling rate greater than the predetermined bit rate. The data samples are applied to a processor which determines the phase and amplitude of the ranging pulses and generates control signals for the terminations to modify the phase or timing and amplitude of the transmissions from the terminations. A locally generated marker pulse may be applied to the incoming data frame before it is sampled in the head-end station, the marker pulse providing a phase and amplitude reference for use by the control processor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a passive optical network, and inparticular to the recovery of information from signals transmitted fromterminations to a head-end station in such a network.

2. Related Art

The present applicant has developed a bit transport system (BTS) for usein a TPON (telephony on a passive optical network) network. In this BTSa head-end station broadcasts time division multiple access (TDMA)frames to all the terminations on the network. The transmitted framesinclude both traffic data and control data. Each termination recognisesand responds to an appropriately addressed portion of the data in thebroadcast frame and ignores the remainder of the frame.

In the upstream direction, each termination transmits data in apredetermined time slot and the data from the different terminations areassembled at the head-end into a TDMA frame of predetermined format.

One feature necessary to such a network is the provision of compensationfor the differing delays and attenuation associated with the differentdistances of the various terminations from the head-end station. To thisend, in the BTS each termination is arranged to transmit a ranging pulsetimed to arrive in a respective predetermined portion of the upstreamTDMA frame. The head-end station is arranged to monitor the timing, i.e.phase, and amplitude of the arrival of the pulse from each of theterminations and to return a servo-control signal to the termination toretard or advance its transmissions as appropriate and to adjust thelaunch power. This active fine ranging enables the BTS to ensure thestability of the upstream TDMA frame and, for example, to compensate forfluctuations in timing and received power due to such effects as changesin the operating temperature of the network. However this places severedemands on the design of the head-end station, requiring the measurementof the timing of received signals to within a fraction of a clock cycleand amplitude to within a fraction of a dB in real time.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provideda method of operating a passive optical network including a head-endstation and a plurality of terminations, the terminations beingresponsive to control signals from the head-end station and the head-endstation being arranged to receive frames having a predetermined bit rateand comprising signals transmitted by the plurality of terminations, themethod including transmitting respective ranging pulses from theterminations to the head-end station, sampling the received rangingpulses at a high sampling rate greater than the said predetermined bitrate and producing corresponding data samples, applying the data samplesto processor means arranged to determine the respective phases of theranging pulses relative to respective expected values and to determinethe respective amplitudes of the ranging pulses, and generatingrespective control signals for the terminations to modify the phase andamplitude of transmissions from the terminations accordingly.

The present invention by sampling the received ranging pulses at a highsampling rate makes it possible to capture the phase and amplitudeinformation from a ranging pulse in a form which can be readilyprocessed to provide the necessary control signals. At the same time,the sampling process effectively provides a snap-shot of the receivedpulse, which carries information relating to the overall shape of thepulse. This further information can be used to implement other controland diagnostic procedures simply by the provision of appropriateadditional software for the processor.

There is residual phase jitter in the TDMA data received at the head-enddue to varying path lengths and clock jitter in the terminations. Withthe method of the present invention, this jitter may be used effectivelyto increase the phase resolution by providing additional sampling pointson the sloped edges of the received ranging pulses. The waveform data isthen integrated and processed by a CPU over a period of time.

Preferably the method further comprises writing the data samples into aplurality of latches connected to a clock signal source arranged toclock the latches at a low clock rate less than the high sampling rate,the clock signal for each latch having a different respective phaseoffset, writing data samples from each latch into a respective serialmemory, and reading data samples from the serial memories into theprocessor means.

Although a high sampling rate is necessary initially to capture all therequired information from the ranging pulse, it is advantageous toconvert the data samples to a lower bit rate, and in particular to thebit rate of the frames before it is accessed by the processor.

Preferably the head-end station includes a masterclock operating at oneof the said low clock and the said high sampling rate and means toderive from the masterclock a clock signal at the other of the said lowclock rate and the said high sampling rate and the plurality of latchesare latched on the leading edge of a pulse of the clock signal at thelow clock rate and the serial memories are latched on the trailing edgeof the said pulse.

According to a further aspect of the present invention, there isprovided a method of operating a passive optical network including ahead-end station and a plurality of terminations, the terminations beingresponsive to control signals from the head-end station and the head-endstation being arranged to receive frames comprising signals transmittedby the plurality of terminations, the method including superimposing onthe received frames at the head-end station a locally generated markerpulse having a phase locked to a masterclock for the system,subsequently sampling a portion of the frame including the marker pulseto produce corresponding data samples, and applying the data samples toa control processor having its own local clock, the control processorbeing arranged to determine characteristics of received signals in thesampled portion of the frame relative to those of the marker pulse.

The further aspect of the present invention is preferably used incombination with the first aspect to analyse ranging pulses. It is nothowever limited to use in this manner and may, for example, be used toderive information from portions of the data frame other than theranging pulses.

According to a still further aspect of the present invention, there isprovided a head-end station for use in a passive optical network systemand arranged to transmit control signals to a plurality of terminations,the terminations being responsive to control signals from the head-endstation, and the head-end station being arranged to receive frameshaving a predetermined bit rate and comprising signals transmitted bythe plurality of terminations, the head-end station including means toreceive respective ranging pulses from the terminations, sampling meansarranged to sample the received ranging pulses at a high sampling rategreater than the said predetermined bit rate and produce correspondingdata samples, processor means responsive to data samples and arranged todetermine the respective phase of the ranging pulses relative torespective expected values and to determine the respective amplitudes ofthe ranging pulses, and control signal generating means arranged togenerate respective control signals to modify the phase and amplitude oftransmissions from respective terminations.

Preferably the head-end station further comprises a plurality of latchesconnected to a clock signal source arranged to clock the latches at alow clock rate less than the said high sampling rate, the clock signalfor each latch having a different respective phase offset, and arespective serial memory for each of the plurality of latches, datasamples being written in use from each latch into the respective serialmemory, and each serial memory being reset by a reset pulse having acorresponding respective phase offset, thereby converting the datasamples from the high sampling rate to the said low clock rate.

Preferably the serial memories comprise dual port serial FIFOs.

Preferably the head-end station further includes marker means disposedupstream of the sampling means and arranged to superimpose a markerpulse of predetermined phase and amplitude on the received frames andthereby provide a reference for use by the processor means.

BRIEF DESCRIPTION OF THE DRAWINGS

A passive optical network in accordance with the present invention willnow be described in detail with reference to the accompanying drawingsin which:

FIG. 1 is a block diagram showing a TPON network;

FIG. 2 is a diagram showing the structure of an upstream TDMA frame;

FIG. 3 is a block diagram of a head-end station;

FIG. 4 is a block diagram showing the input stage in greater detail;

FIG. 5 is a block diagram showing the converter of FIG. 3 in greaterdetail; and

FIG. 6 is a diagram showing the sampling of the ranging pulse.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

A TPON network comprises a head-end station 1, a number of terminations2, and a passive optical fibre network 3 linking the head-end station 1to the terminations 2. Although, for clarity, only three terminationsare shown, in practice many more terminations will be connected to asingle head-end station. Typically the head-end station is located in alocal exchange and the terminations 2 are subscriber stations indomestic or commercial premises or street cabinets in the neighbourhoodof the local exchange.

The head-end station 1 broadcasts data over the fibre network 3 as timedivision multiple access (TDMA) frames having a predetermined format.The frames include control channels addressed to specific ones of theterminations 2 to control, amongst other parameters, the amplitude andphase (timing) of the optical signals transmitted onto the fibre network3 by the terminations 2.

In the upstream direction, each termination 2 transmits data in apredetermined time slot, which data is assembled into a TDMA frame atthe head-end station 1. Since the TPON network is synchronous inoperation it is necessary to control the timing of the terminations bothto compensate for the different delays associated with differentpositions of the terminations on the fibre network 3 and to correct forany variation in the delay and amplitude arising, for example, fromlocal fluctuations in the temperature of the network.

FIG. 2 shows the format of the return frame. Traffic data is transmittedto the head-end station in 80 basic frames BF1 BF80. The basic framesBF1 BF80 are preceded by a header H which includes a phase-2 rangingsection R 720 bits long. Each termination 2 is arranged to transmit ontothe fibre network 3 a ranging pulse timed to arrive at a respectivepredetermined position within the ranging section R. Using the circuitdescribed in further detail below the head-end station 1 determines thephase of each arriving ranging pulse relative to the respectivepredetermined positions of the ranging pulses within the ranging sectionR, and then transmits control signals to the respective termination 2 toretard or advance the timing of the transmission from that termination 2in order to minimize the phase offset between the received data fromthat termination and the intended position of that data within thereturn frame structure.

FIG. 3 shows the structure of the head-end station 1. The incomingoptical signals are received by an input stage 4 which includes aconventional photo-electric detector followed by a linear gain block A(see FIG. 4). The output from the input stage 4 is branched, going bothto a signal recovery stage 5 and to an A/D sampler 6. The sampler 6operates under the control of a CPU 7 to sample any selected portion ofthe incoming signals. In the present example, the sampler 6 is arrangedto sample the portion R of the incoming frame containing the rangingpulses and produce corresponding samples in the form of 8-bit data words(the data). The output from the sampler 6 is taken to a converter 8which converts the data to the lower clock rate of a masterclock for thehead-end station. The CPU 7 calculates from the data the phase andamplitude of the ranging pulses. These parameters are compared withexpected values and, where necessary, control signals generated for acorresponding termination 2 to retard or advance the timing oftransmissions from the termination 2 to minimize the phase offset,and/or to increase or reduce the amplitude of the transmissions untilthe received ranging pulse from that termination 2 has an amplitudefalling within predetermined limits.

FIG. 5 shows the converter in greater detail. The high-speed sampler 6provides an input to the converter 8 comprising 8-bit data words havinga clock rate, in the present example, of 81.92 MHz. The input to theconverter 8 is fed in parallel to four 8-bit latches L. The latches Lare locked to the masterclock with differing phase offsets of 0,90,180,and 270 degrees respectively. The output from each latch is fed to a8-bit dual port serial FIFO of the type known as a video line memoryVLM. In order to ensure that the data written to the video line memoriesis correctly ordered the memories are reset by pulses having apredetermined phase with respect to the write clock. The write clock andreset pulses with appropriate phases are derived from respective shiftregisters, as shown in FIG. 5. The video line memories VLM are polledconsecutively by the CPU 7 so that the data received at the converter 8as four 8-bit data words with a clock rate of substantially 80 MHz areread into the CPU as a single 32-bit word at a lower clock rate ofsubstantially 20 MHz. A write gate signal having the same periodicity asthe received TDMA frames is applied to the write enable inputs WE of thevideo line memories. The write gate is used to select the portion of theTDMA frame which is written into the video line memories.

The head-end station 4 includes a masterclock operating at the lowerclock rate and the components of the head-end station, together with therest of the TPON system are locked to the masterclock. In an alternativearrangement, the head-end station includes a clock operating at the highsampling rate of substantially 80 MHz and the 20 MHz system clock isderived from the 80 MHz clock. However although the control systemsprovided for the head-end station and the network ensure that operationis synchronous at the system clock rate it is not possible to providesimilarly synchronous operation at the higher clock rate used in thehigh-speed sampler. As shown in FIG. 6 this rate is four times themasterclock rate in order to capture the necessary phase and amplitudeinformation from the ranging pulse. In order to provide the phasereference required at the higher sampling rate a marker pulse issuperimposed on the incoming data upstream of the high-speed sampler 6.

As shown in FIG. 4 the input stage includes a marker pulse generator 9,a photodetector 11 and a summing amplifier 10. The marker pulsegenerator 9 produces a marker pulse having a phase locked to themasterclock. The marker pulse is locally summed with the analogue inputsignal at the summing amplifier 10 and so appears in the data outputfrom the sampler 6. The CPU 7 then detects the phase of a given sampledranging pulse with respect to the marker pulse and compares this withthe expected value to determine the appropriate phase control signal.Similarly, the marker pulse provides an amplitude reference for use bythe CPU 7. The CPU 7 includes its own asynchronous clock. The executionof the CPU program is synchronised using 10 mS interrupts (the TDMAframe rate) and data is read from the VLM's at a rate much lower thanthe system clock, controlled by the interrupts.

I claim:
 1. A method of operating a passive optical network including ahead-end station and a plurality of terminations, the terminations beingresponsive to control signals from the head-end station and the head-endstation being arranged to receive frames having a predetermined bit rateand comprising signals transmitted by the plurality of terminations, themethod including:transmitting respective ranging pulses from theterminations to the head-end station, sampling the received rangingpulses at a high sampling rate greater than said predetermined bit rateand producing corresponding data samples, applying the data samples toprocessor means arranged to integrate the data samples to determine therespective phases of the ranging pulses relative to respective expectedvalues and to determine the respective amplitudes of the ranging pulses,and generating respective control signals for the terminations to modifythe phase and amplitude of transmissions from the terminationsaccordingly.
 2. A method of operating a passive optical networkincluding a head-end station and a plurality of terminations, theterminations being responsive to control signals from the head-endstation and the head-end station being arranged to receive frames havinga predetermined bit rate and comprising signals transmitted by theplurality of terminations, the method including:transmitting respectiveranging pulses from the terminations to the head-end station, samplingthe received ranging pulses at a high sampling rate greater than thesaid predetermined bit rate and producing corresponding data samples,applying the data samples to processor means arranged to determine therespective phases of the ranging pulses relative to respective expectedvalues and to determine the respective amplitudes of the ranging pulses.generating respective control signals for the terminations to modify thephase and amplitude of transmissions from the terminations accordingly:writing the data samples into a plurality of latches connected to aclock signal source arranged to clock the latches at a low clock rateless than the high sampling rate, the clock signal for each latch havinga different respective phase offset, writing data samples from eachlatch into a respective serial memory, and reading data samples from theserial memories into the processor means.
 3. A method according to claim2, in which the low clock rate is substantially the same as the bit rateof the received data frames.
 4. A method according to claim 2 includingderiving from a masterclock operating at one of the said low clock rateand the said high sampling rate a clock signal at the other of the saidlow clock rate and the said high sampling rate,latching the plurality oflatches on the leading edge of a pulse of the clock signal at the lowclock rate, and latching the serial memories on the trailing edge ofsaid pulse.
 5. A method of operating a passive optical network includinga head-end station and a plurality of terminations, the terminationsbeing responsive to control signals from the head-end station and thehead-end station being arranged to receive frames comprising signalstransmitted by the plurality of terminations, the methodincluding:superimposing on the received frames at the head-end station alocally generated marker pulse having a phase locked to a masterclockfor the system, subsequently sampling a portion of the frame includingthe marker pulse to produce corresponding data samples, and applying thedata samples to a control processor having its own local clock, thecontrol processor being arranged to determine characteristics ofreceived signals in the sampled portion of the frame relative to thoseof the marker pulse.
 6. A head-end station for use in a passive opticalnetwork system and arranged to transmit control signals to a pluralityof terminations, the terminations being responsive to control signalsfrom the head-end station, and the head-end station being arranged toreceive frames having a predetermined bit rate and comprising signalstransmitted by the plurality of terminations, the head-end stationincluding:means to receive respective ranging pulses from theterminations, sampling means arranged to sample the received rangingpulses at a high sampling rate greater than the said predetermined bitrate and produce corresponding data samples, processor means responsiveto said data samples and arranged to integrate the data samples todetermine the respective phase of the ranging pulses relative torespective expected values and to determine the respective amplitudes ofthe ranging pulses, and control signal generating means arranged togenerate respective control signals to modify the phase and amplitude oftransmissions from respective terminations.
 7. A head-end station foruse in a passive optical network system and arranged to transmit controlsignals to a plurality of terminations, the terminations beingresponsive to control signals from the head-end station, and thehead-end station being arranged to receive frames having a predeterminedbit rate and comprising signals transmitted by the plurality ofterminations, the head-end station including:means to receive respectiveranging pulses from the terminations, sampling means arranged to samplethe received ranging pulses at a high sampling rate greater than thesaid predetermined bit rate and produce corresponding data samples,processor means response to said data samples and arranged to determinethe respective phase of the ranging pulses relative to respectiveexpected values and to determine the respective amplitudes of theranging pulses, control signal generating means arranged to generaterespective control signals to modify the phase and amplitude oftransmissions from respective terminations, a plurality of latchesconnected to a clock signal source arranged to clock the latches at alow clock rate less than the said high sampling rate, the clock signalfor each latch having a different respective phase offset, and arespective serial memory for each of the plurality of latches, datasamples being written in use from each latch into the respective serialmemory, and each serial memory being reset by a reset pulse having acorresponding respective phase offset, thereby converting the datasamples from the high sampling rate to the said low clock rate.
 8. Ahead-end station for use in a passive optical network system andarranged to transmit control signals to a plurality of terminations, theterminations being responsive to control signals from the head-endstation, and the head-end station being arranged to receive frameshaving a predetermined bit rate and comprising signals transmitted bythe plurality of terminations, the head-end station including:means toreceive respective ranging pulses from the terminations, sampling meansarranged to sample the received ranging pulses at a high sampling rategreater than the said predetermined bit rate and produce correspondingdata samples, processor means response to said data samples and arrangedto determine the respective phase of the ranging pulses relative torespective expected values and to determine the respective amplitudes ofthe ranging pulses, control signal generating means arranged to generaterespective control signals to modify the phase and amplitude oftransmissions from respective terminations, a masterclock operating atone of the said low clock rate and the said high sampling rate; means toderive from the masterclock a clock signal at the other of the said lowclock rate and the said high sampling rate, the plurality of latchesbeing latched on the leading edge of the pulse of the clock signal atthe low clock rate and the serial memories being latched on the trailingedge of the said pulse.
 9. A head-end station according to claim 7, inwhich the serial memories comprise dual port serial FIFOs.
 10. Ahead-end station for use in a passive optical network system andarranged to transmit control signals to a plurality of terminations, theterminations being responsive to control signals from the head-endstation, and the head-end station being arranged to receive frameshaving a predetermined bit rate and comprising signals transmitted bythe plurality of terminations, the head-end station including:means toreceive respective ranging pulses from the terminations, sampling meansarranged to sample the received ranging pulses at a high sampling rategreater than the said predetermined bit rate and produce correspondingdata samples, processor means response to said data samples and arrangedto determine the respective phase of the ranging pulses relative torespective expected values and to determine the respective amplitudes ofthe ranging pulses, control signal generating means arranged to generaterespective control signals to modify the phase and amplitude oftransmissions from respective terminations; and marker means disposedupstream of the sampling means and arranged to superimpose a markerpulse of predetermined phase and amplitude on the received frames andthereby provide a reference for use by the processor means.
 11. A methodof determining the phase of ranging pulses returned to a head-endstation from terminations in a TDMA passive optical network and whichpulses exhibit residual jitter and noise, said method comprising thesteps of:sampling said ranging pulses at multiple times within a singlepulse and storing said samples to provide range pulse waveform data;repeating said sampling and storing steps over a period of time toprovide additional range pulse waveform data; and integrating andprocessing said waveform data over a period of time to provide processedreturning ranging pulse waveform data that exhibits reduced jitter andnoise and increased phase resolution.